Abstract

Sediments in the Fig Tree Group, Barberton Mountain Land, South Africa, are interpreted in terms of existing submarine fan-slope models. Midfan beds display close affinities with classical Bouma turbidites; they vary from 20 to 100 cm in thickness and are defined on the basis of upward-decreasing grain sizes and sedimentary structures. Sole marks at the base of beds developed through scouring in the heads of the turbidity currents. Initial sedimentation in the bodies of the currents produced massive graded units under conditions of rapid mass deposition. With slower sedimentation rates, traction deposits developed at the base of beds; these are mainly plane bedded and less commonly contain antidune cross-stratification. Continued waning of the turbidity currents produced lower-flow-regime ripple-drift cross-laminae indicative of high ratios of bedload to suspension load. Overlying horizontally laminated siltstones are interpreted as suspension deposits in the tails of the currents. Background fallout of suspended clays and chemical precipitation of chert followed the passing of individual turbidity currents. In vertical sequence, a-e and complete Bouma cycles are randomly interbedded and signify channel and interchannel deposition on the midfan. Overlying thick shale-banded iron formation–chert sequences, with abundant soft-sediment folding and occasional graywacke interbeds, are interpreted as slope deposits of a prograding fan. These pass upward into massive graywackes with intercalated channel-fill, matrix-supported conglomerates of the feeder channel, which are, in turn, gradational into fluvial conglomerates of the Moodies Group.

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